Photorefractive keratectomy (PRK) is a procedure for laser correction of focusing deficiencies of the eye by modification of corneal curvature. PRK is distinct from the use of laser-based devices for more traditional ophthalmic surgical purposes, such as tissue cutting or thermal coagulation. PRK is generally accomplished by use of a 193 nanometer wavelength excimer laser beam that ablates away corneal tissue in a photo decomposition process. Most clinical work to this point has been done with a laser operating at a fluence level of 120-195 mJ/cm.sup.2 and a pulse-repetition rate of approximately 5-10 Hz. The procedure has been referred to as "corneal sculpting."
Before sculpting of the cornea takes place, the epithelium or outer layer of the cornea is mechanically removed to expose Bowman's membrane on the anterior surface of the stroma. At this point, laser ablation at Bowman's layer can begin. An excimer laser beam is preferred for this procedure. The beam may be variably masked during the ablation to remove corneal tissue to varying depths as necessary for recontouring the anterior stroma. Afterward, the epithelium rapidly regrows and resurfaces the contoured area, resulting in an optically correct (or much more nearly so) cornea.
For ablation to occur, the energy density of the laser beam must be above some threshold value, which is currently accepted as being approximately 60 mJ/cm.sup.2. Such energy densities can be produced by a wide variety of commercially available lasers. For example, a laser could be used that is capable of generating a laser beam of diameter large enough to cover the surface to be ablated, i.e., on the order of 4.5-7.0 millimeters in diameter. However, such laser beams are typically not regular in intensity thereby causing a rough surface ablation. Further, lasers capable of producing such laser beams are typically, large, expensive and prone to failure.
Alternatively, a laser could be used that produces a much smaller diameter laser beam, i.e., on the order of 0.5-1.0 millimeters in diameter. There are several advantages afforded by the smaller diameter laser beam. They can be generated to meet the above noted threshold requirement with a lower energy pulse than that of the larger diameter beam. Further, such smaller diameter laser beams can be produced with a regular intensity while minimizing the variance in pulse-to-pulse energy levels. Finally, lasers producing the smaller diameter laser beam are physically smaller, less expensive and, frequently, more reliable. However, this requires that the position of the small pulses be precisely controlled so that the resulting ablated surface is smoother than that which is produced by the larger laser beam.